Direct sequence spread spectrum (dsss) modulated energy may be used to measure characteristics of a system under test by detecting modulation imposed on the dsss energy by the system. The most typical methods involve phase modulation and amplitude modulation by the system under test. In the case of phase modulation a time delay is imposed, for example when the energy propagates in the system under test at some known velocity. The phase delay would then be recovered by correlation analysis of varying phase delays of the imposed and recovered signals. In the case of amplitude modulation the energy would be changed in amplitude due to the system under test, for example if the energy were dissipated in a system element. The amplitude would then be recovered as proportional to the amplitude of the correlation coefficient of the imposed and recovered signals with zero phase delay between the two. The advantages of using dsss energy as a probe include immunity to interference from other signals, low probability of interference with other signals in the system that exist concurrent with the probe signal, and processing gain which results from the many independent chip measurements represented by the dsss probe energy.
The use of a dsss modulated signal to measure the distance and velocity of objects using ultrasonic energy was disclosed by Wildey in U.S. Pat. No. 7,729,201. Wildey discusses a system wherein a dsss modulated ultrasonic signal is launched toward a surface in a tank, the reflected energy is captured by an ultrasonic transducer, and the resulting phase delay in the dsss ultrasonic signal carries information on the distance from the transducer to the surface. Typically multiple surfaces exist and so multiple reflections represent a simultaneous multiplexed measurement of several target surfaces. Wildey also discloses a system wherein the target objects may be encompassed in a moving fluid where the dsss energy would be Doppler shifted by the relative velocity of the target and the emitting transducer as well as phase shifted by the distance travelled to and from the target object. The dsss modulation method disclosed uses bipolar modulation, i.e. it uses dsss modulation patterns consisting of patterns of +1,−1, levels.
In U.S. Pat. No. 7,069,163, Gunther discloses a system to measure the locations of wire faults in an aircraft by probing with dsss modulated energy. The wire fault, consisting of a short or open circuit, creates an impedance mismatch at the fault. A dsss modulated signal is injected into the wire and is modified by the fault, recovered from the wire and analyzed by correlation with the transmitted dsss pattern to determine phase delay and thus the location of the fault. The system uses orthogonal dsss codes, i.e. bipolar or +1,−1, codes.
It is a goal of the present invention to present a method of probing a system under test using unipolar dsss signals, where unipolar dsss is defined as a set of +1,0, signals as compared to a bipolar dsss set of +1,−1 signals.
It is another goal of the present invention to allow restoration of orthognality to simultaneous unipolar dsss measurements where the recovered energy represents multiple system-modified versions of the externally supplied energy.
It is another goal of the present invention to allow restoration of orthogonality between codes in the multiplexed recovered signal for the case of the probe energy encompassing multiple simultaneous dsss unipolar signals.